Looking at it id say you have probably about 3-5 lbs of rock in there so you'd want roughly 10-12 more pounds but with it being a tall tank it could pose some problems since you don't have much of a footprint

Finally got a decent photo of one. They come and they go, showing up in different places. One comes out of a hole in my live rock, one out of the substrate and this on is coming out of the intake to the filter...

Most of the stuff you stumble upon in saltwater is odd looking. I did freshwatwr for over a decade and some of the stuff ive seen is otherworldly. A good example was yesterday i saw my first bristleworm. Ive known what they are and look like from pictures...but when i saw it i couldnt stop watching it.

Also, a good amount of lr for a tank is 1-1.5 lbs per gallon. O also have a tall tank so you get creative and go up. I bought caribbean lr for how flat and shelf like it is and got a couple pieces of fiji for foundation. This let me make all kinds of overhangs and caves. Dont forget you can always chisel rock to suit your needs and even add pvc pipe in the sandbed as well. Welcome to your new addiction.